We describe a family of calibrations arising naturally on a hyperk\"ahler
manifold $M$. These calibrations calibrate the holomorphic Lagrangian,
holomorphic isotropic and holomorphic coisotropic subvarieties. When $M$ is an
HKT (hyperkaehler with torsion) manifold with holonomy $SL(n, {\Bbb H})$, we
construct another family of calibrations $\Phi_i$, which calibrates holomorphic
Lagrangian and holomorphic coisotropic subvarieties. The calibrations $\Phi_i$
are (generally speaking) not parallel with respect to any torsion-free
connection on $M$.
We present an overview of recent results in locally conformally K\"ahler
geometry, with focus on the topological properties which obstruct the existence
of such structures on compact manifolds.
A mapping class group of an oriented manifold is a quotient of its
diffeomorphism group by the isotopies. We compute a mapping class group of a
hypekahler manifold $M$, showing that it is commensurable to an arithmetic
subgroup in SO(3, b_2-3). A Teichmuller space of $M$ is a space of complex
structures on $M$ up to isotopies.